Conventionally, there have been proposed SOI substrates referred to as Silicon on Quartz (SOQ), Silicon on Glass (SOG) and Silicon on Sapphire (SOS). These SOI substrates can be used as e.g. projectors, high-frequency devices due to the insulation properties and transparency of the handle substrate such as quartz, glass and sapphire.
Primarily, two methods are available for SOI manufacturing technology with regard to bonding.
One of the methods is an SOITEC method. A silicon substrate (donor substrate) into which hydrogen ions have been previously implanted at room temperature is attached to a substrate (handle substrate) serving as a supporting substrate. Subsequently, a bonded wafer is subjected to a heat treatment at high temperature (near 500° C.) to generate a multitude of microscopic gas bubbles called microcavities in an interface of an ion-implanted layer to cause delamination, thereby transferring a silicon thin film to the handle substrate.
The other method is referred to as a SiGen method. Both a surface of a silicon substrate into which hydrogen ions have also been previously implanted and a surface of a handle substrate are plasma-treated to activate surfaces, and then the plasma-treated surfaces are bonded together. Thereafter, the bonded wafer is mechanically delaminated at an interface of a hydrogen ion-implanted layer.
The SOITEC method has a drawback. Since the bonded wafer undergoes a high-temperature heat treatment after bonding, the bonded wafer may crack due to a large difference in thermal expansion coefficient when the silicon substrate is attached to the handle substrate such as quartz and sapphire substrates.
In the SiGen method, the bonded wafer has higher bonding strength compared with the SOITEC method at the time of bonding by surface activation treatment. However, a heat treatment at a temperature of 200° C. or higher is required to bond the substrates. As a result, there may arise such a problem that a bonded wafer breaks due to a difference in the coefficient of thermal expansion between the bonded substrates or untransferred portions are observed in a silicon thin film. This is because while the bonding strength of a bonded interface increases as temperature rises, separation may occur instead due to the warpage of the bonded wafer having different types of the substrates, and therefore, the bonding does not progress uniformly within a plane.
Note that the coefficient of thermal expansion of silicon is 2.6×10−6/K, whereas the coefficients of thermal expansion of quartz and sapphire are 0.56×10−6/K and 5.8×10−6/K, respectively. A difference in the coefficient of thermal expansion (Δα=α(donor)−α(handle)) in the case of SOQ is Δα=2.04×10−6/K (compressive stress is applied to the silicon substrate), and Δα=−3.2×10−6/K (tensile stress is applied to the silicon substrate) in the case of SOS. Thus, the difference is extremely large. If these substrates are directly subjected to a high-temperature treatment after bonding, there arises the problem that the substrates crack or the bonded substrate separates off. Thus, the bonded substrate has the drawback that it is not possible to adopt the SOITEC method or the SiGen method widely used for conventional SOI substrates.